1. Field of the Invention
The invention relates to a technical field of control apparatuses for internal combustion engines that control internal combustion engines equipped with an exhaust gas recirculation (EGR) device.
2. Description of Related Art
A control apparatus of such type in which the EGR amount is controlled by a model estimation control method has been suggested (see, for example, Japanese Patent Application Publication No 2007-113563 (JP 2007-113563 A)).
With the control apparatus for an internal combustion engine disclosed in JP 2007-113563 A, the opening degree of an EGR valve is controlled by using a model estimation control method so that the estimated recirculated gas partial pressure Pirest matches a target value Pirref in order to adequately control the EGR valve opening degree.
In various model estimation control methods including the abovementioned model estimation control method, true dynamics that establishes actual constraints on the EGR amount is most often used after approximation in order to reduce the computational load. In particular, when the true dynamics includes a nonlinear portion, the computational load can be greatly reduced by approximating the nonlinear portion by lines.
Accordingly, in order to obtain a more accurate approximation (that is, the approximation having a high geometric identity) of the true dynamics, the relationship between the dynamics after the approximation (referred to hereinbelow as “approximated dynamics” as appropriate) and true dynamics has been discussed.
However, when only the accuracy of approximation is pursued in a simple manner, the relationship between the approximated values forming the approximated dynamics and the corresponding values in the true dynamics is not necessarily unique. Thus, the approximated values of the approximated dynamics sometimes can easily exceed the true dynamics. When the approximated value exceeds the true dynamics, it means that a virtual constraint aimed at the reduction of computational load is more relaxed than the actual constraint.
Accordingly, in particular when the virtual constraint is more relaxed than the actual constraint, an unrealizable EGR amount is actually allowed as a control target, and therefore the combustion performance of the internal combustion engine can be degraded. Thus, when the EGR amount exceeds a misfire limit as a true constraint, the internal combustion engine can misfire. In other words, According to associated with the apparatus including the device disclosed in JP 2007-113563 A, the optimum EGR amount is difficult to maintain under certain driving conditions of a vehicle.